Simultaneous wafer bonding and interconnect joining

ABSTRACT

Disclosed are a microelectronic assembly of two elements and a method of forming same. A microelectronic element includes a major surface, and a dielectric layer and at least one bond pad exposed at the major surface. The microelectronic element may contain a plurality of active circuit elements. A first metal layer is deposited overlying the at least one bond pad and the dielectric layer. A second element having a second metal layer deposited thereon is provided, and the first metal layer is joined with the second metal layer. The assembly may be severed along dicing lanes into individual units each including a chip.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/076,969, filed on Mar. 31, 2011, now U.S. Pat. No.8,486,758, which claims the benefit of the filing date of U.S.Provisional Patent Application No. 61/424,906 filed Dec. 20, 2010, thedisclosures of which are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to wafer bonding, and in particular, thebonding of wafers together, which may be accompanied by simultaneouslyelectrically interconnecting such wafers.

Wafer-level packaging techniques can be used in a variety ofapplications to simultaneously make microelectronic assemblies whichinclude a plurality of microelectronic elements, such as semiconductorchips stacked one over another with electrical interconnections betweenthe chips. In some cases, wafer-level packaging techniques can be usedto make microelectronic assemblies which include a microelectronicelement having active circuit elements, such as a semiconductor chip,mounted with dielectric or semiconductor element as a packaging layer.Such techniques typically require joining a microelectronic devicewafer, i.e., one having active circuit elements, with another element,which can be another device wafer or a packaging layer (e.g., coverwafer or other wafer) having the same size and shape as the devicewafer.

One of the challenges of such wafer-level processing is to achieve asufficiently planar interface between the wafers and to make reliableelectrical interconnections between contacts on respective wafers.Further improvements in this respect would be desirable.

Size is a significant consideration in any physical arrangement ofchips. The demand for more compact physical arrangements of chips hasbecome even more intense with the rapid progress of portable electronicdevices. Merely by way of example, devices commonly referred to as“smart phones” integrate the functions of a cellular telephone withpowerful data processors, memory and ancillary devices such as globalpositioning system receivers, electronic cameras, and local area networkconnections along with high-resolution displays and associated imageprocessing chips. Such devices can provide capabilities such as fullinternet connectivity, entertainment including full-resolution video,navigation, electronic banking and more, all in a pocket-size device.Complex portable devices require packing numerous chips into a smallspace. Moreover, some of the chips have many input and outputconnections, commonly referred to as “I/O's.” These I/O's must beinterconnected with the I/O's of other chips. The interconnectionsshould be short and should have low impedance to minimize signalpropagation delays. The components which form the interconnectionsshould not greatly increase the size of the assembly. Similar needsarise in other applications as, for example, in data servers such asthose used in internet search engines. For example, structures whichprovide numerous short, low-impedance interconnects between complexchips can increase the bandwidth of the search engine and reduce itspower consumption.

BRIEF SUMMARY OF THE INVENTION

A first aspect of the present invention is a method of forming amicroelectronic assembly including the steps of providing amicroelectronic element having a major surface, and a dielectric layerand at least one bond pad exposed at the major surface, themicroelectronic element containing a plurality of active circuitelements; providing a second element having a coefficient of thermalexpansion of less than 10 ppm/° C., the second element having a majorsurface and a dielectric layer exposed at the major surface; depositinga first metal layer overlying the at least one bond pad and thedielectric layer of the microelectronic element; depositing a secondmetal layer overlying the dielectric layer of the second element; andjoining the first metal layer with the second metal layer.

In accordance with certain embodiments of this first aspect, the stepsof depositing may each include depositing a first stage including atleast one of copper or aluminum onto at least one of the microelectronicelement or the second element. The microelectronic element may be achip, and the second element may have an area substantially the same asan area of the chip. The step of depositing the first metal layer mayinclude depositing a metal over substantially the entire major surfaceof the microelectronic element, and the method may further includeremoving a portion of the metal such that the metal extends to apredetermined height above the major surface of the microelectronicelement. The method may further include removing the metal directlyoverlying the major surface of the microelectronic element at a gapbetween the at least one bond pad and the dielectric layer. The gap maybe large enough to provide a relief volume sufficient to compensate fora sum of the total variation in co-planarity of the top surfaces of thefirst metal layer overlying the at least one bond pad and the dielectriclayer on the microelectronic element, and the total variation inco-planarity of the top surfaces of the second metal layer overlying thedielectric layer on the second element and at least one bond pad exposedat the major surface thereof.

The step of joining may include heating at least one of the first andsecond metal layers to a temperature between about 50 and 300° C. Atleast one of the first and second metal layers may include at least aportion which is exothermic and thermally-activated through applicationof heat, and the step of joining may include heating the at least aportion of the exothermic metal layer to thermally activate such layer.The second element may be a microelectronic element containing aplurality of active circuit elements and may include at least one bondpad exposed at the major surface. At least one of the elements mayinclude a through silicon via electrically connected with the at leastone bond pads and extending from the major surface of such elementtoward a second surface of the element remote from the major surface. Athrough silicon via may extend through the microelectronic and secondelements and may be electrically connected with a bond pad of themicroelectronic element and a bond pad of the second element. The stepof joining may include juxtaposing the at least one bond pad of themicroelectronic element with the at least one bond pad of the secondelement and heating the first and second metal layers to a joiningtemperature. The at least one bond pad on each of the elements mayinclude a plurality of bond pads aligned in a plurality of rows. The atleast one bond pad on each of the elements may include a plurality ofbond pads aligned adjacent a periphery of the respective major surface,and the dielectric layer may overlie a central region of the majorsurface.

The dielectric layer of at least one of the elements may be compressibleto absorb any dimensional tolerances. The height of the at least onebond pad above the major surface of the microelectronic element maydiffer from the height of the dielectric layer above the major surfaceof the microelectronic element. At least one of the steps of depositingmay include depositing a reflow metal, and the step of joining mayinclude heating the reflow metal to a temperature that causes the reflowmetal to melt. The reflow metal may be selected from the groupconsisting of tin, solder, indium, gold, and any combination thereof.The steps of depositing may include depositing copper, and the step ofjoining may include applying heat and pressure between the elements suchthat the copper overlying the microelectronic element and the copperoverlying the second element fuse together. The method may furtherinclude depositing a layer of gold on the copper overlying at least oneof the elements, and the step of joining may include heating the gold toa temperature at which the gold diffuses into the copper. The step ofdepositing the first and second metal layers may include depositing abase metal and a layer of gold overlying the base metal, and the step ofjoining may include applying heat and pressure to the elements until thefirst and second metal layers fuse together.

A second aspect of the present invention is a microelectronic assemblyincluding a microelectronic element having a major surface, and adielectric layer and at least one bond pad exposed at the major surface,the microelectronic element containing a plurality of active circuitelements; a second element having a coefficient of thermal expansion ofless than 10 ppm/° C., the second element having a major surface, and atleast one bond pad and a dielectric layer exposed at the major surfacethereof; a first metal layer overlying the at least one bond pad and thedielectric layer of the microelectronic element, wherein a gap is formedin the first metal layer between the at least one bond pad and thedielectric layer; a second metal layer overlying the at least one bondpad and the dielectric layer of the second element; and wherein firstportions of the first and second metal layers overlying the dielectriclayers are joined together and second portions of the first and secondmetal layers overlying the at least one bond pads and being separatefrom the first portions are joined together so as to mechanically andelectrically connect the microelectronic element with the secondelement.

In accordance with certain embodiments of this second aspect, themicroelectronic element may be a chip, and the second element may be anarea substantially the same as an area of the chip. The gap may be largeenough to provide a relief volume sufficient to compensate for a sum ofthe total variation in co-planarity of the top surfaces of the firstmetal layer overlying the at least one bond pad and the dielectric layeron the microelectronic element, and the total variation in co-planarityof the top surfaces of the second metal layer overlying the at least onebond pad and the dielectric layer on the second element. At least one ofthe first and second metal layers may include at least a portion whichis exothermic and thermally-activated through application of heat.

The second element may be a microelectronic element containing aplurality of active circuit elements. At least one of the elements mayinclude a through silicon via electrically connected with the at leastone bond pads and extending from the major surface of such elementtoward a second surface of the element remote from the major surface. Athrough silicon via may extend through the microelectronic and secondelements and may be electrically connected with a bond pad of themicroelectronic element and a bond pad of the second element. The atleast one bond pad on each of the elements may include an array of bondpads arranged in a plurality of rows. The at least one bond pad on eachof the elements may include a plurality of bond pads arranged adjacent aperiphery of such element, and the dielectric layer may overlie acentral region of the major surface. A gap may be formed in the secondmetal layer between the at least one bond pad and the dielectric layer.

The dielectric layer of at least one of the elements may be compressibleto absorb any dimensional tolerances. The height of the at least onebond pad above the major surface of the microelectronic element maydiffer from the height of the dielectric layer above the major surfaceof the microelectronic element. At least one of the metal layers mayinclude a reflow metal selected from the group consisting of tin,solder, indium, gold, and any combination thereof. Each of the first andsecond metal layers may include a layer of metal which is wettable bythe reflow metal, wherein the reflow metal overlies the wettable metallayer. The first and second metal layers may include copper. Theassembly may further include a layer of gold overlying at least one ofthe first and second metal layers. The first and second metal layers maynot directly overly the respective major surfaces of the elements.

A third aspect of the present invention is a microelectronic assemblyincluding a microelectronic element having a major surface and adielectric layer exposed at the major surface; and a second elementhaving a major surface and a dielectric layer exposed at the majorsurface; the major surfaces of the first and second wafers confrontingone another with a plurality of electrically metal elements disposedbetween the dielectric layers, the metal elements being electricallyisolated from the microelectronic element, wherein the metal elementsjoin the first and second wafers with one another.

In accordance with certain embodiments of this third aspect, the secondelement has a coefficient of thermal expansion of less than 10 ppm/° C.The microelectronic element may contain a plurality of active circuitelements. The second element may be a microelectronic element containinga plurality of active circuit elements and may include at least one bondpad exposed at the major surface. The microelectronic element may be achip, and the second element may be an area substantially the same as anarea of the chip. On each element, a gap may be formed along the majorsurface between each metal element and adjacent portion of thedielectric layer, and the gap may be large enough to provide a reliefvolume sufficient to compensate for a sum of the total variation inco-planarity of top surfaces of the metal elements and the dielectriclayer overlying the major surface of the microelectronic element, andthe total variation in co-planarity of top surfaces of the metalelements and the dielectric layer overlying the major surface of thesecond element. The dielectric layer of at least one of the elements maybe compressible to absorb any dimensional tolerances.

A fourth aspect of the present invention is a system including astructure as described above and one or more other electronic componentselectrically connected to the structure. In accordance with certainembodiments of this second aspect, the system may further include ahousing, the structure and the other electronic components being mountedto the housing.

Further aspects of the invention provide systems which incorporatemicroelectronic structures according to the foregoing aspects of theinvention, composite chips according to the foregoing aspects of theinvention, or both in conjunction with other electronic devices. Forexample, the system may be disposed in a single housing, which may be aportable housing. Systems according to preferred embodiments in thisaspect of the invention may be more compact than comparable conventionalsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a microelectronic element in accordancewith a first embodiment of the present invention.

FIG. 2 is an enlarged view of a portion of the element shown in FIG. 1.

FIG. 3 is a top view of the element shown in FIG. 1.

FIG. 4 is a sectional view of the element shown in FIG. 1 with a thinmetal layer disposed thereon.

FIG. 5 is an enlarged view of a portion of the element shown in FIG. 4.

FIG. 4A is a sectional view of an element in accordance with anotherembodiment of the present invention.

FIG. 5A is a sectional view of a portion of the element shown in FIG. 4Awith a thin metal layer disposed thereon.

FIG. 6 is a top view of the element shown in FIG. 4.

FIG. 7 is a sectional view of the element shown in FIG. 4 with areflowable metal layer disposed thereon.

FIG. 8 is an enlarged view of a portion of the element shown in FIG. 7.

FIG. 9 is a top view of the element shown in FIG. 7.

FIGS. 10 and 11 are sectional views of an assembly of two elements inaccordance with an embodiment of the present invention.

FIGS. 12 and 13 are sectional views of assemblies in accordance with thepresent invention having through silicon vias.

FIG. 14 is a sectional view of an assembly of two elements in accordancewith another embodiment of the present invention.

FIG. 15 is a sectional view of an assembly of two elements in accordancewith another embodiment of the present invention.

FIG. 16 is a schematic depiction of a system according to one embodimentof the invention.

DETAILED DESCRIPTION

In accordance with one embodiment of the present invention, FIGS. 1-3depict a microelectronic element 100, e.g., a device wafer embodyingactive circuit elements, has a major surface 102 and a second surface104 remote from major surface 102. A dielectric layer 120 and at leastone bond pad 110 are exposed at major surface 102. Dielectric layer 120is preferably comprised of a material that may be compressible to absorbany dimensional tolerances with regard to bond pads 110 above majorsurface 102. In certain embodiments, the height of at least one of bondpads 110 above major surface 102 differs from the height of dielectriclayer 120 above major surface 102, as shown more clearly in FIG. 2.

Microelectronic element 100 may be a wafer or a semiconductor chiphaving a plurality of active circuit elements, or a portion of a wafercontaining a plurality of semiconductor chips. In another example,microelectronic element 100 may be reconstituted wafer or panelincluding a plurality of active chips arranged in an array and heldtogether for processing simultaneously. Shown more clearly in FIG. 3 isa portion of a wafer containing four semiconductor chips 111 attachedtogether at dicing lanes 112. Bond pads 110 may be arranged in an arrayincluding, for example, being arranged in one or more rows. For example,bond pads 110 may be disposed in rows adjacent the dicing lanes alongthe periphery 125 of each chip. In certain embodiments, dielectric layer120 may be disposed within the peripheral bond pads to overlie a centralregion of major surface 102.

FIGS. 4-6 depict microelectronic element 100 having a metal layer 130overlying bond pads 110 and dielectric layer 120. The metal layer 130can be relatively thin. As shown in FIGS. 7-9, a joining metal such as areflowable metal layer 150 can be deposited to overlie metal layer 130.When the metal layer 150 is reflowable, it may include tin, solder,indium, gold, or any combination thereof. When the metal layer 150 is areflowable metal, the underlying metal layer 130 can be wettable by thereflowable metal. In a particular example, the wettable metal layer 130can include a layer of copper or copper alloy, aluminum or aluminumalloy, or a combination thereof. Such layer or layers can form a basestructure of the metal layer 130. Such metal layer 130 may furtherinclude a layer of nickel overlying the base structure. In anotherexample, the metal layer 130 may include such layer of nickel, and alayer of gold overlying the nickel layer.

Metal layer 150 may not directly overlie major surface 102, and a gap140 may exist in metal layer 150 between bond pad 110 and dielectriclayer 120. A gap 140 may exist between each adjacent pair of bond pads110 and dielectric layers 120. Each individual bond pad 110 anddielectric layer can be isolated on major surface 102.

FIGS. 10 and 11 depict microelectronic element 100 being assembled withanother element 200, typically to form a wafer-level assembly 300, orother assembly 300 including one or more active chips bonded andelectrically interconnected with another active chip or other element.Element 200 may be substantially similar to microelectronic element 100having active circuit elements, or may be a passive element or waferhaving passive circuit elements. Element 200 typically is made ofsemiconductor material and typically is a semiconductor device waferembodying active circuit elements, e.g., transistors, diodes, etc.However, in a particular embodiment, element 200 can be a passive oressentially blank semiconductor wafer or other element which can includea base of glass or ceramic material, or other material having a CTE lessthan 10 ppm/° C. Element 200 may be as shown in FIGS. 4A and 5A, such asan element having a coefficient of thermal expansion of less than 10ppm/° C. Element 200 has a major surface 202, a second surface 204remote from major surface 202, and a dielectric layer 220 exposed atmajor surface 202. In certain embodiments, such as the one depicted inFIGS. 10 and 11, element 200 may be a microelectronic element containinga plurality of active circuit elements and may include at least one bondpad 210. A thin metal layer 230 overlies dielectric layer 220, as shownin FIG. 5A, and also overlies bond pads 210, as shown in FIG. 10. Areflowable metal layer 250 is deposited to overlie metal layer 230. Themetal layers 230 and 250 can be as described above for metal layers 130,150 of the microelectronic element or wafer 100. A gap 240 may exist inmetal layer 250 between bond pad 210 and dielectric layer 220, or in thecase of a passive element, in between adjacent portions of dielectriclayer 220.

In forming assembly 300, elements 100 and 200 are provided with at leastelement 100 being an active microelectronic element. Metal layers 150and 250 are deposited over the respective elements. This step may becarried out in different ways. For example, a metal may be depositedover substantially the entire major surface 102, 104 of the respectiveelement 100, 200, covering any bond pads 110, 210 and/or dielectriclayers 120, 220 disposed thereon. A portion of the metal may then beremoved such that the metal extends to a predetermined height abovemajor surface 102, 104. The metal directly overlying major surface 102,104 may then be removed at gap 140, 240 between bond pads 110, 210 anddielectric layers 120, 220. Further, each step of depositing therespective metal layer 150, 250 may include depositing a first stageincluding a metal which can include copper or aluminum, for example.

Prior to joining, bond pads 110 of element 100 are juxtaposed with bondpads 210 of element 200, as shown in FIG. 10. Together, bond pads 110and 210 may comprise a plurality of electrically isolated metal elementsthat are eventually disposed on their respective major surfaces 102, 202between dielectric layers 120, 220. Metal layers 130, 230 above thedielectric material (or the isolated metal elements) may be electricallyisolated from internal elements within the respective element 100, 200or, alternatively, may be usable for connection to a source of power orground. As shown in FIG. 11, elements 100 and 200 are then joined to oneanother with metal layer 150 joined to metal layer 250 to form assembly300, such as by heating the reflow metals to a temperature that causesthem to melt and fuse together. In one embodiment, at least one of metallayers 150, 250 is heated to a temperature between about 50 and 300° C.For example, joining of metal layers of solder, tin, indium or gold, orcombination thereof, can usually be performed at temperatures below 300°C. At least one of metal layers 150 and 250 may include at least aportion which is exothermic and thermally-activated through applicationof heat, and the step of joining may include heating the at least aportion of the exothermic metal layer to thermally activate such layer.Any dimensional tolerances are preferably accounted for and absorbed byat least one of dielectric layers 120, 220 being compressible.

Gaps 140 and 240 can be large enough to provide a relief volumesufficient to compensate for a sum of the total variation inco-planarity of the top surfaces of metal layers 150 and 250 overlyingthe respective bond pads 110 and 210 and dielectric layers 120 and 220.As can be seen in FIG. 11, certain portions of the combined metal layers150 and 250 may bulge out into gaps 140, 240 as seen at areas 152.Alternatively, portions of such combined metal layers can appear to haveconcave surfaces adjacent gaps 140, 240 as shown at areas 154. It isalso possible that portions of the combined metal layers 150 and 250have essentially straight and vertical edges as seen at 156. Thus, thejoining of elements 100 and 200 is aided by the reflowable metal layers150 and 250 being able to electrically connect while potentiallyoverflowing into gaps 140, 240 due to any dimensional differentiationrelated to elements 100 and 200.

In a variation of assembly 300, metal layers 150 and 250 may comprisecopper instead of a reflowable metal and one or both metal layers 130,230 in some cases may be omitted. Joining such layers includes applyingheat and pressure between elements 100, 200 such that the copper layersfuse together. A layer of gold may overlie at least one of metal layers150 and 250, which may be heated to a temperature at which the golddiffuses into the copper at the joining interface between the metallayers 150, 250.

In another embodiment, the step of depositing metal layers 150, 250includes depositing a base metal and a layer of gold overlying the basemetal. Heat and pressure are then applied to the elements until metallayers 150, 250 fuse together.

In one embodiment, after bonding elements 100 and 200 together, assembly300 may be severed along dicing lanes, one such lane being denoted byline 301, into individual microelectronic assemblies or units.Typically, each unit includes an active chip, i.e., a chip embodyingactive circuit elements, and a corresponding portion of the element 200,which may or may not include another active chip. Assembly 300 may befurther constructed as shown in FIG. 12 such that at least one ofelements 100 and 200 includes a through via 302 electrically connectedwith a bond pad and extending through the thickness of such element. Forexample, a through via 302 extends through element 100 and provides anelectrically conductive element therein which is electrically connectedto a bond pad 110. Similarly, an electrically conductive through via 302can extend through element 200 and be electrically connected with bondpad 210. An electrically conductive through via 303 may extend throughboth elements 100 and 200 and be electrically connected with a bond pad110 of element 100 and a bond pad 210 of element 200, as shown in FIG.13.

Various ways of making the electrically conductive through vias exist,which can be as described in U.S. Provisional Application Nos.61/419,033 and 61/419,037, United States Patent Publication No.2008/0246136, or U.S. application Ser. Nos. 12/842,717 and 12/842,651,for example, the disclosures of which are incorporated herein byreference.

In certain embodiments, only one of metal layers 150, 250 is presentprior to joining elements 100 and 200. This construction is shown inFIG. 14, where element 1100 includes metal layer 1150 overlying metallayer 1130, and element 1200 includes as its top layer only metal layer1230, with no other metal layer overlying same.

In another embodiment, shown in FIG. 15, an element 2200 can be aninterposer, cover wafer or blank wafer, which has no exposed bond pads.Elements 2100, 2200 each include a dielectric layer 2120, 2220,respectively, and a metal layer 2130, 2230 overlying dielectric layers2120, 2220, respectively. Bond pads 2110 overlying major surface 2102 ofelement 2100 are adjacent an exposed portion of major surface 2202 ofelement 2200. Metal layers 2130, 2230, which may only overlie dielectriclayers 2120, 2220 and not bond pads 2110, can be electricallyinterconnected through the joined reflowable metal layers 2150, 2250.

By the foregoing processing, it is possible for the metal layers 130(FIG. 11), or metal layers 2130 (FIG. 15) on respective wafers orelements to be electrically isolated from one another, and from thewafer or element on which they are provided both before and afterbonding the wafers or elements together. Alternatively, some or all ofthe metal layers 130 or 230 on a wafer or element can be usable forconnection to a source of power or ground. For example, a metal layer130 or 230 on a wafer can extend onto or be electrically connected withground bond pads of the same wafer which provide ground connections tothe chips of such wafer. In still another example, the metal layer 130,or 230 on a wafer can extend onto or be electrically connected withpower bond pads of the same wafer which provide power source connectionsto the chips of such wafer. In yet another example, some portions of themetal layers 130, 230 of each microelectronic assembly can extend ontoor be electrically connected with ground bond pads, and other portionsof the metal layers 130, 230 of each microelectronic assembly can extendonto or be electrically connected with power bond pads. In these cases,the metal layer of each chip of both wafers, e.g., metal layers 130,230, and the reflowable metal 150 (FIG. 11) between them in eachassembly would be available for connection to a ground terminal or to apower terminal of a microelectronic assembly in which the chips areincorporated.

By the processing described in the foregoing in accordance with one ormore of the above-described embodiments, the techniques provided hereintypically can compensate for nonplanarity in bonding interface. In oneexample, the joining or reflowable metal layers 150, 250 can have athickness of 1 micron on the metal layers 130, 230 and the bond pads110, 210 of the respective wafers. In such embodiment, the nominaljoined thickness of the metal layer 150 (e.g., FIG. 11) is 2 microns.The techniques provided herein can compensate for a total variation inthe planarity of the bonding interface across the entire dimensions ofthe wafers to an extent greater than 0.5 microns. In another example,when the joining or reflowable metal layers 150, 250 have appropriatethicknesses, the techniques herein can compensate for even greaternonplanarity, such as 3 microns.

The structures discussed above provide extraordinary three-dimensionalinterconnection capabilities. These capabilities can be used with chipsof any type. Merely by way of example, the following combinations ofchips can be included in structures as discussed above: (i) a processorand memory used with the processor; (ii) plural memory chips of the sametype; (iii) plural memory chips of diverse types, such as DRAM and SRAM;(iv) an image sensor and an image processor used to process the imagefrom the sensor; (v) an application-specific integrated circuit (“ASIC”)and memory. The structures discussed above can be utilized inconstruction of diverse electronic systems. For example, a system 900 inaccordance with a further embodiment of the invention includes astructure 906 as described above in conjunction with other electroniccomponents 908 and 910. In the example depicted, component 908 is asemiconductor chip whereas component 910 is a display screen, but anyother components can be used. Of course, although only two additionalcomponents are depicted in FIG. 16 for clarity of illustration, thesystem may include any number of such components. The structure 906 asdescribed above may be, for example, a composite chip as discussed aboveor a structure incorporating plural chips. In a further variant, bothmay be provided, and any number of such structures may be used.Structure 906 and components 908 and 910 are mounted in a common housing901, schematically depicted in broken lines, and are electricallyinterconnected with one another as necessary to form the desiredcircuit. In the exemplary system shown, the system includes a circuitpanel 902 such as a flexible printed circuit board, and the circuitpanel includes numerous conductors 904, of which only one is depicted inFIG. 16, interconnecting the components with one another. However, thisis merely exemplary; any suitable structure for making electricalconnections can be used. The housing 901 is depicted as a portablehousing of the type usable, for example, in a cellular telephone orpersonal digital assistant, and screen 910 is exposed at the surface ofthe housing. Where structure 908 includes a light-sensitive element suchas an imaging chip, a lens 911 or other optical device also may beprovided for routing light to the structure. Again, the simplifiedsystem shown in FIG. 16 is merely exemplary; other systems, includingsystems commonly regarded as fixed structures, such as desktopcomputers, routers and the like can be made using the structuresdiscussed above.

As these and other variations and combinations of the features discussedabove can be utilized without departing from the present invention, theforegoing description of embodiments should be taken by way ofillustration rather than by way of limitation of the invention.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. (canceled)
 2. A method of forming a microelectronic assemblycomprising the steps of: providing a microelectronic element having amajor surface, and a dielectric layer and at least one bond pad exposedat the major surface, the microelectronic element containing a pluralityof active circuit elements, wherein a height of the at least one bondpad above the major surface differs from a height of the dielectriclayer above the major surface, the dielectric layer of at least one ofthe elements being compressible to absorb any dimensional tolerances;providing a second element having a coefficient of thermal expansion ofless than 10 ppm/° C., the second element having a major surface and adielectric layer exposed at the major surface of the second element;forming a first metal layer such that a first portion of the first metallayer overlies the at least one bond pad and a second portion of thefirst metal layer overlies the dielectric layer of the microelectronicelement; forming a second metal layer overlying the dielectric layer ofthe second element; and joining the first metal layer with the secondmetal layer.
 3. The method of claim 2, wherein the step of joiningincludes juxtaposing the at least one bond pad of the microelectronicelement with at least one bond pad exposed at the major surface of thesecond element, thereby simultaneously bonding the microelectronic andsecond elements and electrically interconnecting the juxtaposed bondpads.
 4. The method of claim 2, wherein the steps of forming eachinclude depositing a first stage including at least one of copper oraluminum onto at least one of the microelectronic element or the secondelement.
 5. The method of claim 2, wherein the step of forming the firstmetal layer includes forming a metal over substantially the entire majorsurface of the microelectronic element, and further comprising the stepof removing a portion of the metal such that the metal extends to apredetermined height above the major surface of the microelectronicelement.
 6. The method of claim 5, further comprising removing the metaldirectly overlying the major surface of the microelectronic element at agap between the at least one bond pad thereof and the dielectric layerthereof.
 7. The method of claim 6, wherein the gap is large enough toprovide a relief volume sufficient to compensate for a sum of the totalvariation in co-planarity of top surfaces of the first metal layeroverlying the at least one bond pad and the dielectric layer on themicroelectronic element, and the total variation in co-planarity of topsurfaces of the second metal layer overlying the dielectric layer on thesecond element and at least one bond pad exposed at the major surfacethereof.
 8. The method of claim 2, wherein the step of joining includesheating at least one of the first and second metal layers to atemperature between about 50 and 300° C.
 9. The method of claim 2,wherein at least one of the first and second metal layers includes atleast a portion which is exothermic and thermally-activated throughapplication of heat, and the step of joining includes heating the atleast a portion of the exothermic metal layer to thermally activate suchlayer.
 10. The method of claim 2, wherein the second element includes atleast one bond pad exposed at the major surface thereof, and wherein atleast one of the elements includes a through silicon via electricallyconnected with the at least one bond pads of such element and extendingfrom the major surface of such element toward a second surface of suchelement remote from the major surface of such element.
 11. The method ofclaim 10, wherein a through silicon via extends through themicroelectronic and second elements and is electrically connected with abond pad of the microelectronic element and a bond pad of the secondelement.
 12. The method of claim 2, wherein at least one of the steps offorming includes depositing a reflow metal, and the step of joiningincludes heating the reflow metal to a temperature that causes thereflow metal to melt.
 13. The method of claim 2, wherein the steps offorming include depositing copper, and the step of joining includesapplying heat and pressure between the elements such that the copperoverlying the microelectronic element and the copper overlying thesecond element fuse together.
 14. The method of claim 13, furthercomprising forming a layer of gold on the copper overlying at least oneof the elements, and the step of joining includes heating the gold to atemperature at which the gold diffuses into the copper.
 15. The methodof claim 2, wherein the step of forming the first and second metallayers includes depositing a base metal and a layer of gold overlyingthe base metal, wherein the step of joining includes applying heat andpressure to the elements until the first and second metal layers fusetogether.
 16. A microelectronic assembly comprising: a microelectronicelement having a major surface, and a compliant layer and at least onebond pad exposed at the major surface, the microelectronic elementcontaining a plurality of active circuit elements; a second elementhaving a coefficient of thermal expansion of less than 10 ppm/° C., thesecond element having a major surface and a compliant layer exposed atthe major surface thereof; a first metal layer overlying the at leastone bond pad and the compliant layer of the microelectronic element,wherein a gap is formed in the first metal layer between the at leastone bond pad and the compliant layer; a second metal layer overlying thecompliant layer of the second element; and wherein first portions of thefirst and second metal layers overlying the compliant layers are joinedtogether so as to mechanically and electrically connect themicroelectronic element with the second element.
 17. The assembly ofclaim 16, wherein the second element has at least one bond pad exposedat the major surface thereof, the second metal layer overlies the atleast one bond pad, wherein second portions of the first and secondmetal layers overlying the at least one bond pads and being separatefrom the first portions are joined together.
 18. The assembly of claim16, wherein the gap is large enough to provide a relief volumesufficient to compensate for a sum of the total variation inco-planarity of the top surfaces of the first metal layer overlying theat least one bond pad and the compliant layer on the microelectronicelement.
 19. The assembly of claim 16, wherein the height of the atleast one bond pad above the major surface of the microelectronicelement differs from the height of the compliant layer above the majorsurface of the microelectronic element.
 20. A microelectronic assemblycomprising: a microelectronic element having a major surface and acompliant layer exposed at the major surface; and a second elementhaving a major surface and a compliant layer exposed at the majorsurface of the second element; the major surfaces of the microelectronicand second elements confronting one another with a plurality of metalelements disposed between the compliant layers, the metal elements beingelectrically isolated from the microelectronic element, wherein themetal elements join the microelectronic and second elements with oneanother, wherein on each of the microelectronic element and the secondelement, a gap is formed along the major surface between one or more ofthe metal elements and an adjacent portion of the compliant layer,wherein the gap is large enough to provide a relief volume into which areflowable metal can flow from between the compliant layers of themicroelectronic and second elements, the relief volume being sufficientto compensate for a sum of the total variation in co-planarity of topsurfaces of the metal elements and the compliant layer overlying themajor surface of the microelectronic element, and the total variation inco-planarity of top surfaces of the metal elements and the compliantlayer overlying the major surface of the second element.
 21. Theassembly of claim 20, wherein the microelectronic element is a chip, andthe second element has an area substantially the same as an area of thechip.